The controversy as to whether there is a specific attractive intermolecular force between chlorine atoms, of the charge-transfer or donor-acceptor type, is resolved using various analyses of experimental crystal structure data and theoretical calculations. The occurrence of Cl-Cl intermolecular contacts which are shorter than would be expected from the conventional isotropic van der Waals radius is shown to be most common in the crystal structures of fully or highly chlorinated hydrocarbons, and thus a consequence of close packing of anisotropic atoms, rather than evidence for a specific attractive force. Intermolecular perturbation theory calculations on the Cl-Cl interactions within the chloromethane dimer show that the charge-transfer contribution to the intermolecular energy is negligible, the electrostatic forces are weak, and the repulsive wall is anisotropic. Calculations on the electrostatic interactions between other chlorinated hydrocarbons show that these results will also apply to other Cl-Cl interactions. Thus a realistic anisotropic model for the repulsion, dispersion, and electrostatic forces between chlorinated hydrocarbons should be capable of predicting the observed crystal structures with "short" Cl-Cl intermolecular separations.

Exact quantum reactive scattering calculations in the collision energy range 1-250 meV have been carried out for both the isotopic product channels of the title system. The dynamical studies compares an ab initio potential energy surface (PES) recently appeared in the literature (J. Chem. Phys., 2008, 129, 011103) with other phenomenological PESs. Vibrational branching ratios, cross sections and rate constants are presented and compared with molecular beam scattering experiments as well as with chemical kinetics data. In particular, the agreement of the vibrational branching ratios with experimental measurements is improved with respect to previous studies on other PESs, mainly because of the presence of a broad peak in the HF(v' = 3) integral cross section completely absent in the previous simulations. This feature, observed by molecular beam experiments, is the fingerprint of a new reaction mechanism operative in the dynamics described by the new PES. A conjecture for its origin, able to explain many of its characteristic aspects, is analyzed and discussed.

The achievement of extensive and meaningful molecular dynamics simulations requires both the detailed knowledge of the basic features of the intermolecular interaction and the representation of the involved potential energy surface in a simple, natural and analytical form. This double request stimulated us to extend to ion-molecule systems a semiempirical method previously introduced for the description of weakly interacting atom-molecule aggregates and formulated in terms of atomic species-molecular bond interaction additivity. The method is here applied to the investigation of the prototypical M(+)-C6H6 systems (M = Li, Na, K, Rb and Cs) and some of its predictions are tested against accurate ab initio calculations. Such calculations have been performed by employing the MP2 method and large basis sets, privileging the description of the metal atoms. The agreement between potential energy scans semiempirically obtained and ab initio results is good for all the investigated geometries, thus showing that the adopted representation is in general able to reproduce all the main features of the potential energy surface for these systems. The role of the various noncovalent interaction components, as a function of the geometry and of the intermolecular distance in the M(+)-C6H6 complexes, is also investigated for a more detailed assessment of the results of the semiempirical method.

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